This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The overall goal of our research is to understand the three-dimensional structures that guide pre-mRNA splicing, and to thereby provide a foundation to develop treatments for inherited diseases associated with errors in splice site recognition. During the critical early stages of splice site choice, the essential splicing factor U2AF cooperatively recognizes the pre-mRNA splice site as a complex with Splicing Factor 1 (SF1). This interaction is regulated by phosphorylation of key SF1 domain that contributes to U2AF binding, but lacks detectable homology with known structures. The primary aim of the proposed work is to determine the crystal structure of this key SF1 domain. The novel structure of this SF1 domain would shed light on the means of action by this important splicing factor. In addition, we recently obtained crystals of the phosphorylated state of the SF1 domain, which would reveal signal-dependent conformational changes.